S100A7 is highly expressed in ER?- invasive carcinoma, as well as in high grade ductal carcinomas in situ (DCIS), and therefore may play an important role in breast cancer progression and metastasis. Our preliminary studies indicate that S100A7 has differential effects on ER?+ and ER?- cells; it enhances growth and metastasis in ER?- and inhibits growth in ER?+ breast cancer cells. However, it remains to be determined if and how S100A7 modulates differential effects in ER?+ and ER?- breast cancer subtypes. Our central hypothesis is that the differential effects of S100A7 are a result of two different pathways: in ER?-, it may modulate the tumor microenvironment by binding to receptor for advance glycation end products (RAGE) and transactivating EGFR; in ER?+ cells, it may regulate the 2-catenin pathway. The mortality of patients is significantly caused by the invasive characteristics of ER?-, especially in triple-negative breast cancers, and the development of drug resistance in ER?+ breast cancers; as a result, understanding how S100A7 may modulate differential effects in ER?- and ER?+ breast cancers is of fundamental importance. To this end, we will use an innovative, multi-disciplinary approach to analyze the role and molecular mechanisms of S100A7, taking advantage of transgenic and knockout mouse model systems. In Aim 1, we will further analyze S100A7 expression in breast cancer tissue microarrays in different grades and subtypes, especially triple-negative breast cancers. In Aim 2, we will further characterize the role of S100A7 in modulating the growth and metastasis of ER?+ and ER?- cells in in vivo mouse models. In Aim 3, we will analyze the role of S100A7 in breast cancer progression and metastasis in transgenic and knockout mouse model systems. Finally, in Aim 4, we will delineate the S100A7-mediated molecular mechanisms that enhance growth and metastasis of ER?- cells and inhibit cell proliferation and migration of ER?+ cells. Insight gained from these studies may help in developing novel and innovative therapies for highly invasive ER?- and drug resistant ER?+ breast cancers.